U.S. patent number 10,374,531 [Application Number 15/317,289] was granted by the patent office on 2019-08-06 for motor control device and electric power steering system using said motor drive circuit.
This patent grant is currently assigned to Hitachi Automotive Systems, Ltd.. The grantee listed for this patent is Hitachi Automotive Systems, Ltd.. Invention is credited to Takuro Kanazawa, Nobuyasu Kanekawa, Ryoichi Kobayashi, Tomonobu Koseki.
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United States Patent |
10,374,531 |
Kanekawa , et al. |
August 6, 2019 |
Motor control device and electric power steering system using said
motor drive circuit
Abstract
The purpose of the present invention is not only to reduce
ripple current but also to improve operation efficiency by reducing
heat generation. When n is defined as an integer of 2 or more, a
motor control device of the present invention drives n sets of
windings by n sets of inverters. The motor control device is
characterized in that when said n sets of inverters output high
power, at least one or more of said n sets of inverters are set to
have an output duty cycle of 100%.
Inventors: |
Kanekawa; Nobuyasu (Tokyo,
JP), Kobayashi; Ryoichi (Hitachinaka, JP),
Koseki; Tomonobu (Hitachinaka, JP), Kanazawa;
Takuro (Tokyo, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
Hitachi Automotive Systems, Ltd. |
Hitachinaka-shi, Ibaraki |
N/A |
JP |
|
|
Assignee: |
Hitachi Automotive Systems,
Ltd. (Hitachinaka-shi, JP)
|
Family
ID: |
55018896 |
Appl.
No.: |
15/317,289 |
Filed: |
May 11, 2015 |
PCT
Filed: |
May 11, 2015 |
PCT No.: |
PCT/JP2015/063410 |
371(c)(1),(2),(4) Date: |
December 08, 2016 |
PCT
Pub. No.: |
WO2016/002340 |
PCT
Pub. Date: |
January 07, 2016 |
Prior Publication Data
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|
|
|
Document
Identifier |
Publication Date |
|
US 20170117830 A1 |
Apr 27, 2017 |
|
Foreign Application Priority Data
|
|
|
|
|
Jun 30, 2014 [JP] |
|
|
2014-133499 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02P
21/22 (20160201); H02P 27/08 (20130101); H02P
21/50 (20160201); H02P 29/60 (20160201); H02P
27/085 (20130101); H02P 25/22 (20130101); B62D
5/046 (20130101); H02P 6/10 (20130101); B62D
5/04 (20130101) |
Current International
Class: |
H02P
27/08 (20060101); H02P 25/22 (20060101); B62D
5/04 (20060101); H02P 29/60 (20160101); H02P
21/00 (20160101); H02P 6/10 (20060101); H02P
21/22 (20160101) |
Field of
Search: |
;318/599,811,800,801,798 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
|
|
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|
|
2 061 145 |
|
May 2009 |
|
EP |
|
60-128852 |
|
Jul 1985 |
|
JP |
|
2009-100587 |
|
May 2009 |
|
JP |
|
2010-161846 |
|
Jul 2010 |
|
JP |
|
2012-161154 |
|
Aug 2012 |
|
JP |
|
2014-50150 |
|
Mar 2014 |
|
JP |
|
WO 2013/105225 |
|
Jul 2013 |
|
WO |
|
Other References
International Search Report (PCT/ISA/210) issued in PCT Application
No. PCT/JP2015/063410 dated Aug. 11, 2015 with English translation
(Three (3) pages). cited by applicant .
Extended European Search Report issued in counterpart European
Patent Application No. 15815400.5 dated Jan. 31, 2018 (Ten (10)
pages). cited by applicant .
Cho et al., "Optimal Design of Heatsink for 3 Phase Voltage Source
Inverter System by Using Average Method," Modern Applied Science,
Jan. 16, 2010, pp. 114-125, XP055441979. cited by
applicant.
|
Primary Examiner: Duda; Rina I
Attorney, Agent or Firm: Crowell & Moring LLP
Claims
The invention claimed is:
1. A motor control device for a motor, comprising: a set of n coils
driven by a set of n inverters, n being defined as an integer of 2
or more; and two redundant control devices, each of the redundant
control devices being configured to individually control the motor,
such that upon failure of one of the two redundant control devices,
the other of the two redundant control devices is configured to
continue controlling the motor, wherein at least one inverter in
the set of n inverters has an output duty cycle of 100% when output
of the set of n inverters is higher than a given value, the at
least one inverter switching another inverter of the set of n
inverters with a predetermined frequency, and the motor device is
configured to operate at a given cycle, the given cycle being
1/(the predetermined frequency), the given cycle is less than a
thermal time constant of each of the at least one inverter and the
another inverter.
2. The motor control device according to claim 1, further
comprising three phase to two phase converters, each three phase to
two phase converter being directly interposed between the motor and
one of the set of n inverters.
3. The motor control device according to claim 2, wherein m is
defined as an integer equal to or less than the n, an output duty
cycle of m inverters is set to 100% when a target value of a total
duty cycle of output of the set of n inverters is m.times.100/n %
or more.
4. The motor control device according to claim 2, wherein the at
least one inverter has the output duty cycle of 100% is switched
with the predetermined frequency.
5. The motor control device according to claim 4, wherein the set
of n inverters has an inverter having the output duty cycle set at
100%, the inverter being switched at the predetermined
frequency.
6. The motor control device according to claim 3, wherein duty
cycles of output of the set of n inverters are differentiable by
the total duty cycle.
7. The motor control device according to claim 3, wherein a first
derivative of the total duty cycle of the duty cycles of output of
the set of n inverters is zero or more.
8. The motor control device according to claim 2, wherein when duty
cycles of output of at least two inverters are less than 100%,
switching timing of output of the at least two inverters has a
phase difference.
9. The motor control device according to claim 2, further
comprising a slope control function that controls a switching
waveform of an output waveform to be a moderate gradient when all
of the set of n inverters are normal, and controls the switching
waveform of the output waveform of a remaining inverter to be a
steep gradient when failure occurs in the set of n inverters.
10. An electric power steering system comprising: a set of n coils
driven by a set of n inverters, n being defined as an integer of 2
or more; a steering wheel; a rotation shaft attached to the
steering wheel; a torque sensor attached to the rotation shaft; a
steering mechanism; a motor; and two redundant control devices,
each of the redundant control devices being configured to
individually control the motor, such that upon failure of one of
the two redundant control devices, the other of the two redundant
control devices is configured to continue controlling the motor,
wherein the steering mechanism is steered by the rotation shaft,
the steering mechanism or the rotation shaft has steering force
assisted by the motor, the motor control device controls the motor,
at least one inverter in the set of n inverters has an output duty
cycle of 100% when output of the set of n inverters is higher than
a given value, the at least one inverter switching another inverter
of the set of n inverters with a predetermined frequency, the motor
device is configured to operate at a given cycle, the given cycle
being 1/(the predetermined frequency), the given cycle is less than
a thermal time constant of each of the at least one inverter and
the another inverter.
11. The electronic power steering system according to claim 10,
further comprising three phase to two phase converters, each three
phase to two phase converter being directly interposed between the
motor and one of the set of n inverters.
Description
TECHNICAL FIELD
The present invention relates to a motor control device that uses a
semiconductor switching element to convert electric power supplied
from a power source.
BACKGROUND ART
Progress of automated control requires higher safety and
reliability of electronic control devices. In the event of an
abnormality, immediate detection of the abnormality is required to
stop operation in order to secure safety of electronic control
devices.
In addition, it is required not only to reliably stop operation in
the event of failure for safety, but also to continue the operation
after safety is secured. For example, since an electric power
steering system has been used in a large-size vehicle with larger
weight as improved in performance, stopping its operation in the
event of failure causes large steering force to be manually
generated. Thus, the operation needs to be continued even in the
event of failure after safety is secured.
As a technique for continuing operation of an electric power
steering system even in the event of failure, for example, PTL 1
(JP 2012-161154 A) discloses a technique for allowing a motor to
include a pair of coils so that the pair of coils is driven by a
pair of inverters. PTL 1 also discloses a technique for reducing
ripple current by applying an offset to a duty cycle of PWM
modulation of the pair of inverters, as well as for reducing
imbalance of heat loss by changing the offset depending on a
steering state.
CITATION LIST
Patent Literature
PTL 1: JP 2012-161154 A
SUMMARY OF INVENTION
Technical Problem
The techniques disclosed in PTL 1 above need a pair of inverters to
enable operation to continue in the event of failure, and may cause
a cost increase. Thus, it is further desirable that an advantage
surpassing the cost increase be enjoyed even in normal time without
failure. PTL 1 above discloses the technique for reducing ripple
current by applying an offset to a duty cycle of PWM modulation of
the pair of inverters. The present invention has been made in light
of the above-mentioned problem, and an object thereof is not only
to reduce ripple current, but also to improve operation efficiency
by reducing heat generation.
Solution to Problem
To solve the problem described above, the present invention is a
motor control device in which a set of n coils is driven by a set
of n inverters, where n is defined as an integer of 2 or more, and
at least one inverter in the set of n inverters has an output duty
cycle of 100% when output of the set of n inverters is high.
Advantageous Effects of Invention
As described above, the present invention can reduce switching loss
of an inverter that drives a motor and improve efficiency.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a block diagram of a motor-inverter system according to
the present embodiment.
FIG. 2 is a waveform chart illustrating an example of a motor drive
waveform when Dall is 50% or more.
FIG. 3 is a map diagram is a state where Da is 2Dall and Db is 0
when Dall is less than 50%, and Da is 100% and Db is (2Dall-100)%
when Dall is 50% or more.
FIG. 4 is a waveform chart illustrating an example of a motor drive
waveform when Dall is less than 50%.
FIG. 5 is a map diagram illustrating a state where Da and Db each
equal Dall when Dall is less than 50%, and Da is 100% and Db is
(2Dall-100)% when Dall is 50% or more.
FIG. 6 is a waveform chart of motor drive by a pair of
inverters.
FIG. 7 is a map diagram illustrating an example in which sudden
change of Da and Db is avoided in FIG. 5.
FIG. 8 is a map diagram illustrating an example in which weak
monotonic increase (.delta.Da/.delta.Dall is zero or more, and
.delta.Db/.delta.Dall is zero or more) is applied to Da and Db with
respect to Dall.
FIG. 9 is a waveform chart of motor drive by a pair of
inverters.
FIG. 10 is a graph of loss when on-resistance Ron of a MOSFET is
large.
FIG. 11 is a graph of loss when on-resistance Ron of the MOSFET is
small.
FIG. 12 is a graph of loss when on-resistance Ron of the MOSFET is
small.
FIG. 13 illustrates change over time of respective duty cycles D1
and D2 of a first inverter 100-1 and a second inverter 100-2.
FIG. 14 is a waveform chart illustrating an example in which one of
the first inverter 100-1 and the second inverter 100-2 fails, and
only the other inverter drives a motor
FIG. 15 is a waveform chart illustrating an example in which one of
the first inverter 100-1 and the second inverter 100-2 fails, and
only the other inverter drives the motor 2.
FIG. 16 is a block diagram illustrating an example of a control
device 200 for achieving the present embodiment.
FIG. 17 is a first system block, diagram of a motor control device
according to the present embodiment.
FIG. 18 is a second system block diagram of the motor control
device according to the present embodiment.
FIG. 19 is a block diagram of an electric power steering system
according to the present embodiment.
DESCRIPTION OF EMBODIMENTS
Before description of the present invention based on an example, a
principle of the present invention will be described.
When a motor is driven by a plurality of (n) inverters, a duty
cycle equivalent to that when the motor is driven by a single
inverter (hereinafter referred to as a total duty cycle Dall) is an
average value of a duty cycle Di (i is an ID number of an inverter)
of output of each of the inverters, or a value obtained by
Expression 1 below, if rating of each of the inverters is the same.
Dall=.SIGMA..sub.i=1.sup.nDi/n (Expression 1)
If an output duty cycle of one inverter is 100%, Dall is 100/n % or
more. Thus, in the present method, an appropriate Dall can be
acquired only in the case where a target total duty cycle Dall is
100/n % or more, even if an output duty cycle of one inverter is
100%.
In consideration of providing two inverters, an appropriate Dall
can be acquired only in the case where target Dall is 50% or more,
even if an output duty cycle of one inverter is 100%.
The method described above can reduce switching loss by setting a
duty cycle of at least one of a pair of inverters to 100%. The
switching loss mentioned here includes not only loss associated
with turning on and off of a switching element, but also loss
caused by a junction potential difference of a body diode in a
period until the switching element is turned on for synchronous
switching at the time when current is returned.
In addition, alternate switching between inverters each having a
duty cycle of 100% can reduce imbalance of heat loss (ohmic loss
P=Ron.times.I{circumflex over ( )}2, where P is heat generation,
Ron is on-resistance of a switching element, and I is current).
Hereinafter, embodiments of the present invention will be described
with reference to drawings.
First Embodiment
FIG. 1 is a block diagram of a motor-inverter system according to
the present embodiment.
A motor 2 includes a pair of coils, and is driven by each of a
first inverter 100-1 and a second inverter 100-2. While each of the
coils of the motor 2 may be connected to form a .DELTA. connection
or a Y connection (star connection), the present embodiment can be
applied to both of the .DELTA. connection and the Y connection. In
addition, while a case of including a pair of inverters is
described in the present embodiment, the invention according to the
present embodiment can be applied to even a motor control device
including a pair of inverters or more.
That is, under a condition where n is defined as the number of
inverters and in is defined, as an integer equal to or less the n,
an output duty cycle of m inverters is set to 100% when a target
value of a total duty cycle of output of a set of n inverters is
m.times.100/n % or more.
In the present embodiment, the first inverter 100-1 applies Du1,
Dv1, and Dw1 to a first coil of the motor 2. Likewise, the second
inverter 100-2 applies a waveform of a duty cycle of each of Du2,
Dv2, and Dw2 to a second coil, of the motor 2.
FIG. 2 is a waveform chart of motor drive in the present
embodiment.
As illustrated in FIG. 2, Du1, Dv1, and Dw1 formed by the first
inverter 100-1 are switched to form a cycle of 1/f1 in the order
from Da being a duty cycle of 100% to Db being a duty cycle of
other than 100%. Likewise, Du2, Dv2, and Dw2 formed by the second
inverter 100-2 are switched to form a cycle of 1/f1 in the order
from Db to Da. In addition, a cycle of a PWM waveform is indicated
as 1/f2.
In the example described above, since there is no switching
operation in the duty cycle Da (=100%) no switching loss is
generated and loss can be reduced accordingly. In addition, since
the inverters are alternately driven at the duty cycle Da (=100%),
heat generation can be prevented from concentrating at one of the
inverters.
From a different viewpoint, it can be also thought that f1 is a
switching frequency during Da (=100%), and f2 is a switching
frequency during Db. Since f1 is less than f2, switching loss can
be significantly reduced during Da (=100%) in which f1 is a
switching frequency.
It is desirable that f2 be a frequency within an audible range or
more than the audible range, and that a cycle 1/f1 be less than a
thermal time constant of each of the first inverter 100-1 and the
second inverter 100-2. Specifically, it is desirable that the cycle
1/f1 be less than a thermal time constant of the inverters.
Second Embodiment
Each of FIGS. 3, 5, 7, and 8 illustrates an example of a target
total duty cycle Dall and an example of the duty cycles Da and Db
to be assigned to each inverter.
FIG. 3 is a map diagram illustrating a state where Da is 2Dall and
Db is 0 when Dall is less than 50%, and Da is 100% and Db is
(2Dall-100)% when Dall is 50% or more. FIG. 4 is a waveform chart
illustrating an example of a motor drive waveform when Dall is less
than 50%. FIG. 2 is a waveform chart illustrating an example of a
motor drive waveform when Dall is 50% or more.
FIG. 5 is a map diagram illustrating a state where Da and Db each
equal Dall when Dall is less than 50%, and Da is 100% and Db is
(2Dall-100)% when Dall is 50% or more. When Dall is less than 50%,
as illustrated in FIG. 6, the first inverter 100-1 and the second
inverter 100-2 each operate in the same duty cycle, and have a
waveform with a reversed phase to reduce ripple current flowing
into a power source and a capacitor connected to the power source.
When Dall is 50% or more, a motor drive waveform illustrated in
FIG. 2 is obtained as with the example of FIG. 3.
FIG. 7 is a map diagram illustrating an example in which sudden
change of Da and Db is avoided in FIG. 5. The example shows Da and
Db that are gradually increased or reduced from Dall of x1 just
before Dall becomes 50%, under a condition where Da and Db each
equal Dall, so that Da becomes 100% and Db becomes 0% when Dall is
50%, and Da becomes 100% and Db becomes (2Dall-100)% when Dall is
50% or more. When Dall is in a range between x1 and 50%, a motor
drive waveform has Da and Ph that are alternately switched as
illustrated in FIG. 9.
If a control loop has conversion from Dall to Da and Db, or Da and
Db have characteristics of causing sudden change, operation of a
control system may become unstable due to an error in Dall, which
may cause hunting or the like. The present example eliminates a
possibility that operation of the control system becomes unstable,
by avoiding sudden change of Da and Db. For example, Dall of x1 is
45% or the like.
FIG. 8 is a map diagram further illustrating an example in which
weak monotonic increase (.delta.Da/.delta.Dall is zero or more, and
.delta.Db/.delta.Dall is zero or more) is applied to Da and Db with
respect to Dall. Da and Db each equal Dall in a region where Dall
is less than x1. Db is x1 and Da is increased to (2Dall-x1) in a
region where Dall is more than x1 but less than x2. Da is 100% and
Db is (2Dall-100)% in a region where x2 is less than Dall.
FIG. 17 is a first system block diagram of a motor control device
according to the present embodiment. FIG. 18 is a second system
block diagram of the motor control device accord rig to the present
embodiment.
In a system configuration including duplex (redundant) control
devices 200-1 and 200-2, as illustrated in FIGS. 17 and 18, to
avoid influence of failure in the control device 200-1 or 200-2, a
conversion error in A/D converters 208-1 and 208-2 illustrated in
FIG. 18, or the like, may cause a difference between Dall target
values of the control devices 200-1 and 200-2.
If the Dall target value includes an error as described above,
control may be unstable in a case were conversion from Dall to Da
and Db is not a monotonic increase, but has decrease as illustrated
in FIG. 7. For example, if the error causes the Dall target value
to be less than X1 in one of the control devices 200-1 and 200-2
that outputs Da, and causes the Dall target value to be X1 or more
in the other thereof that outputs Db, a gradient of Da becomes 1, a
gradient of Db becomes extremely negative, and a gradient of
(Da+Db) becomes negative. Thus, a control gain that is essentially
positive may become negative to deteriorate stability of the
control system.
Then, as shown in the present example, applying weak monotonic
increase to Da and Db with respect to Dall enables control
operation to be stably performed even in the system configuration
including the duplex (redundant) control devices 200-1 and 200-2,
as illustrated in FIGS. 17 and 18, to avoid influence of failure in
the control device 200-1 or 200-2. From a viewpoint of continuous
control, it is desirable to apply strict monotonic increase
(.delta.Da/.delta.Dall is more than 0, and .delta.Db/.delta.Dall is
More than 0) to Da and Db with respect to Dall. However, from a
viewpoint of stability of control, applying weak monotonic increase
is sufficient. That is, the weak monotonic increase allows duty
cycles of output of a set of n inverters to be differentiable by a
total duty cycle. The strict monotonic increase allows a first
derivative of a total duty cycle of duty cycles of output of a set
of n inverters to be zero or more.
FIG. 10 is a graph of loss when on-resistance Ron of MOSFET is
large. A dotted line corresponds to a conventional method without
using the present embodiment, a dashed line corresponds to a
control method illustrated in FIG. 3, and a solid line corresponds
to a control method illustrated in FIG. 8.
In the control method illustrated in FIG. 3, it is found that ohmic
loss (i{circumflex over ( )}2Ron) increases near Da of 100% and
loss increases as compared to the conventional method in a region
where Dall is less than 50%. In a region where Dall is more than
60%, it is found that loss is reduced more in the methods of FIGS.
3 and 8 than in the conventional method.
FIGS. 11 and 12 each are a graph of loss when on-resistance Ron of
a MOSFET is small. In the control method illustrated in FIG. 3, it
is found that ohmic loss (i{circumflex over ( )}2Ron) increases
near Da of 100% and loss increases as compared to the conventional
method in a region where Dall is less than 50%, as illustrated in
FIG. 11. In a region where Dall is more than 60%, it is found that
loss is reduced more in the methods of FIGS. 3 and 8 than in the
conventional method, but that reduction effect of loss in FIG. 8 is
inferior to the conventional method in a region near Dall of 60%.
In this region, it is found that low loss characteristics are
sacrificed due to the monotonic increase. As illustrated in FIG.
12, loss is reduced in the region where Dall is more than 60% by a
method of FIG. 5.
FIG. 13 illustrates change over time of respective duty cycles D1
and D2 of the first inverter 100-1 and the second inverter 100-2.
Although a duty cycle can also be changed stepwise as illustrated
by a solid line, a control level difference due to a control error
or the like can be prevented from occurring by gradually changing
the duty cycle as illustrated by a broken line.
Third Embodiment
FIGS. 14 and 15 each are a waveform chart illustrating an example
in which one of the first inverter 100-1 and the second inverter
100-2 fails, and only the other inverter (the first inverter 100-1
in the present embodiment) drives a motor 2.
The switching waveform has a moderate gradient as illustrated by a
solid line during normal time to prevent electromagnetic noise from
occurring, and has a steep gradient as illustrated by a broken line
in the event of failure of the inverter to reduce switching loss.
The gradient of a switching waveform can conventionally be achieved
by capacitance of a gate circuit of a MOSFET in an output stage,
and a time constant defined by driving force of a driving circuit.
That is, the moderate gradient can be achieved by reducing driving
force of a driving circuit during normal time, and the steep
gradient can be achieved by increasing the driving force of the
driving circuit in the event of failure of the inverter.
A driving duty cycle may be formed of Da and Db as illustrated in
FIG. 14, or may be formed of Dall as illustrated in FIG. 15. In
addition, the duty cycle may be formed of 2Dall (an upper limit is
100%) immediately after failure of an inverter, and may be
gradually reduced to be formed of Dall. According to this method, a
remaining inverter is temporarily driven by a doubled current
immediately after the failure of the inverter in order to prevent
total torque from being halved due to the failure of the
inverter.
Fourth Embodiment
FIG. 16 is a block diagram illustrating an example of a control
device 200 for achieving the present embodiment. Currents of each
phase Iu1, Iv1, and Iw1, and Iu2, Iv2, and Iw2 of the first
inverter 100-1 and the second inverter 100-2, are respectively
converted into d-axis currents and q-axis currents Id1 and Iq1, and
Id2 and Id2 by three-phase/two-phase converters 205 and 206, and
then combined to form a d-axis current Id and a q-axis current Iq.
The d-axis current Id and the q-axis current Iq are subtracted from
current command values Id* and Iq* calculated by a current command
value calculator 201 to form error signals .delta.Id and .delta.Iq,
respectively. Target voltages Vd and Vq are generated by a
controller 202, and then a two-phase/three-phase converter 203
converts the target voltages Vd and Vq into voltage target values
of each phase Vu, Vv, and Vw. Subsequently, a duty calculator 204
forms target duty cycles of each phase Duall, Dvall, and Dwall.
Subsequently, Duall, Dvall, and Dwall are converted into duty
cycles Du1, Dv1, and Dw1, and duty cycles Du2, Dv2, and Dw2,
distributed, to the first inverter 100-1 and the second inverter
100-2, respectively, for each phase, by a duty distributor 205
according to FIGS. 3, 5, 7, and 8. Accordingly, the first inverter
100-1 and the second inverter 100-2 are driven by a PWM.
FIGS. 17 and 18 each illustrate an example having redundant control
devices 200-1 and 200-2. Particularly, FIG. 18 illustrates an
example in which the three-phase/two-phase converters 205 and 206
also have redundant converters 205-1 and 205-2, and duplex
converters 206-1 and 206-2, respectively. In addition, an A/D
converter for converting a current value and a torque signal also
has redundant converters 208-1 and 208-2.
According to the present example described above, since the
redundant control devices 200-1 and 200-2 are provided, failure of
the control device does not cause both of the first inverter 100-1
and the second inverter 100-2 to stop operating.
Fifth Embodiment
FIG. 19 is a configuration diagram of an electric power steering
system according to the present embodiment. The electric power
steering system includes the first inverter 100-1 and the second
inverter 100-2, and also includes a steering wheel 11, a rotation
shaft 16 attached to the steering wheel 11, a torque sensor 12
attached to the rotation shaft 16, a steering mechanism 17,
microprocessors 100-1 and 100-2, and a motor 8. A phase current
detection signal 14 and a total current detection signal 15 are
input to the control device 200.
The steering mechanism 17 controls a direction of wheels 18 steered
by the rotation shaft 16. The steering mechanism 17 or the rotation
shaft 16 has steering force assisted by the motor 2. The first
inverter 100-1 and the second inverter 100-2 are controlled on the
basis of output of the control device 200 to drive the motor 2.
Thus, in the electric power steering system according to the
present example, a duty cycle of each of the first inverter 100-1
and the second inverter 100-2 is optimized to reduce heat loss.
REFERENCE SIGNS LIST
2 motor 11 steering wheel 12 torque sensor 14 phase current
detection signal 15 total current detection signal 15 16 rotation
shaft 17 steering mechanism 18 wheel 100-1 first inverter 100-2
second inverter 200 control device Da duty cycle Db duty cycle Dall
total duty cycle
* * * * *